The present disclosure relates to the field of printing. In particular, the invention relates to an ink-deposition device suitable for depositing ink on a target surface by conversion of digital images into ink images, to a printing system comprising the device, and to methods of using the same.
Digital Printing
Digital printing techniques have been developed that allow a printer to receive instructions directly from a computer without the need to prepare printing plates as in traditional offset printing. Such processes broadly fall into two categories: direct printing, in which the ink is directly deposited on the final printing substrate (e.g. sheets or webs of fibrous or non-fibrous materials as paper, cardboard, plastic, etc.), and indirect printing, in which the ink is first deposited on an intermediate transfer member and therefrom to the final printing substrate.
Inkjet and bubble jet printers, which are commonly used in home and office settings, are examples of direct printing systems. In these processes droplets of ink are sprayed onto a final substrate in an image pattern. In general, the resolution of such processes is limited due to wicking by the inks into porous paper substrates or other fibrous material. To reduce this phenomenon, fibrous substrates, such as paper, generally require specific coatings engineered to absorb the liquid ink in a controlled fashion or to prevent its penetration below the surface of the substrate. Using specially coated substrates is, however, a costly option that is unsuitable for certain printing applications, especially for commercial printing. Furthermore, the use of coated substrates creates its own problems in that the surface of the substrate remains wet and additional costly and time consuming steps are needed to dry the ink, so that it is not later smeared as the substrate is being handled, for example stacked or wound into a roll. Furthermore, excessive wetting of the substrate causes cockling and makes printing on both sides of the substrate (also termed duplex printing) difficult, if not impossible.
Using an indirect printing technique overcomes many problems associated with inkjet printing directly onto the printing substrate. It allows the distance between the surface of the intermediate image transfer member and the inkjet print head to be maintained constant and reduces wetting of the final substrate, as the ink can be dried on the surface of the intermediate transfer member (ITM) before being applied to it, thus reducing or preventing bleeding of the ink image beneath the surface and into the structure of the printing substrate. Consequently, the final image quality of the ink film on the substrate is less affected by the physical properties of the substrate.
Aqueous based inks have a number of distinct advantages whether considered for direct or indirect printing systems. For the latter systems, silicone coated transfer members are generally preferred, since this facilitates transfer of the dried image to the final substrate. The respective chemical and/or physical properties of the ink and of the target surface it may interact with (e.g. an ITM or a printing substrate) are known to affect the resulting print quality. As noted above, the surface of the printing substrate may be treated (e.g. coated) to mitigate any undesired interaction between the ink and for instance the paper. Similarly, the surface of the intermediate transfer member may be treated or conditioned to improve its interaction with the ink.
Half-Toning
Half-toning is well-known in the art, in particular in the field of printing. Generally speaking, a digital image (e.g. a grayscale image) is first converted to a binary image, and the binary image is printed onto a target surface (e.g. of paper or another substrate), thereby converting the binary image into an ink-image.
FIG. 1 illustrates a prior-art technique for half-toning. In step S101, both (i) a ‘target’ multi-level digital image and (ii) a pixel-image mask, are stored in computer readable medium. Generally speaking, a computer readable medium may include storage media or memory media (i.e. volatile or non-volatile) such as magnetic or flash or optical media, e.g. disk or CD-ROM, volatile or non-volatile media such as RAM, ROM, etc. as well as transmission media or signals such as electrical, electromagnetic or digital signals conveyed via a communication medium such as network and/or wireless links.
In step S105, the pixel-image mask is applied to the target multi-level digital image to yield the target binary image. Typically, this application is a mathematical transformation performed by electrical circuitry (e.g. of a digital computer).
In step S109, ink-droplets are deposited on the target surface according to the contents of the binary-image, thereby converting the target binary image into an ink-image. Referring to FIG. 2, it is noted that droplets are only deposited at locations on the surface that correspond to a respective location within the binary-image having a ‘1’ value. However, once the ink is deposited on the surface, the deposited ink may behave so that the shape of the ink-image does not exactly match that of the binary-image. In the example of FIG. 2, two square-clusters of pixels are printed from the binary image—however, the corresponding ink image is somewhat ‘distorted’ and is not exactly square.
FIG. 3 illustrates a system for performing the method of FIG. 1. Within a computer storage (e.g. volatile or non-volatile computer memory) is stored the pixel-mask. A binary image is formed from an image according to the content of the pixel-mask. The binary-image is converted by an ink-deposition device into an ink-image on a target surface. The ink-formations deposited on the target surface make up the ink-image.